A. Field of the Invention
The present invention relates to a new and improved valve actuator with manual override.
B. Description of the Prior Art
Typical valve actuators that include a manual override and torque limit provision employ a clutch mechanism between the manual override and the valve actuator. Alternative actuators require actual disengagement of the actuator from the valve and manual operation of the valve stem, assuming the stem is accessible to actuate the valve manually. Typical prior art valves and actuators are, however, manufactured in a manner such that manual operation of the driving apparatus is difficult to deform due to limited accessibility. It is therefore highly desirable that a manual override capability be provided in a valve actuator wherein the input need not be disengaged nor a complex clutch mechanism required.
Some prior art actuators provide manual drive of an epicyclic gear train adjacent to a power source or driven input. In this construction, due to the high gear ratio of the succeeding actuator train, it is necessary to use an additional "step up" gear to provide any reasonable hand-wheel operation. In other prior art actuators, an epicyclic train is driven by a prime mover through a spur gear set. With this construction, manual actuation of any part of the epicyclic train will produce reverse rotation of the drive motor and hence requires an auxiliary motor brake or other additional braking means. Another prior art actuator is disclosed in U.S. Pat. No. 4,261,224.
The actuator disclosed hereinafter overcomes these disadvantages through the use of selectably irreversible input drive gear sets, and manual actuation of an epicyclic train very near to the output shaft.